This article describes the effect of the lateral size of graphene oxide (GO) on the humidity sensing properties of a GO-based sensor. The GO size effects on the humidity sensing performance were evaluated on gold electrodes drop-coated with either ultralarge graphene oxide UGO (lateral size = 47.4 ± 22.2 μm) or small-size graphene oxide SGO sheets (lateral size = 0.8 ± 0.5 μm). The in-plane conductance obtained from the UGO and SGO electrodes was found to increase by, respectively, four orders of magnitude and by three orders of magnitude upon a relative humidity RH change from 7 to 100 %. The maximal sensitivity (S) values of the UGO and SGO humidity sensors were determined to be SUGO = 4339 ± 433 and SSGO = 1982 ± 122. The GO size clearly influenced the overall proton conductivity, as evidenced by the activation enthalpy (Ea) required for proton conduction in UGO and SGO sheets: Ea (UGO) = 0.63 eV, Ea (SGO) = 1.14 eV. The UGO humidity sensor exhibited an excellent device performance with a high sensitivity and an ultrafast response/recovery time (0.2/0.7 s). Good stability was observed, with a variation of only ±4.6% over five days. The resistive-type UGO humidity sensor was capable of sensing the moisture on a fingertip at a distance of 0.5 mm with a sensitivity of 17.4 and a response/recovery time of 0.6 s/1.3 s. The excellent device performance of the UGO humidity sensor also permitted the determination of the position of a fingertip by detecting the fingertip moisture, hence offering a great potential for touchless display position interface applications.